Do you think it's weird to have sex with a new partner, who is not the father of the child during pregnancy?

My ex husband dumped me when we found out that we were expecting, he also cheated on me.

We're getting divorced of course. When I was 5 months along I found someone.. we started seeing each other. It felt really nice to be cared about again... a few weeks later we had sex. It never bothered him I was pregnant (he has two of his own with his ex).

I told my best friend about him of course and she thought I was crazy for having sex with him while being pregnant from my ex... is it?